Process for reducing diethylene glycol formation in poly(ethylene terephthalate)prepolymer

ABSTRACT

FORMATION OF CONTAMINANT DIETHYLENE GLYCOL BY-PRODUCT DURING MANUFACTURE OF POLY(ETHYLENE TEREPHTHALATE) PREPOLYMER BY ESTER EXCHANGE OF DIMETHLY TEREPHTHALATE AND ETHYLENE GLYCOL IN THE PRESENCE OF A CATALYST MIXTURE IS MINIMIZED BY INCLUDING LITHIUM ACETATE DIHYDRATE AS AN ESSENTIAL INGREDIENT IN THE CATALYST MIXTURE WHEN THE CATALYST MIXTURE CONSISTS OF ZINC ACETATE DIHYDRATE OR ANTIMONY TRIOXIDE, ALONE OR TOGETHER, AS THE CATALYST OR CATALYST FOR SAID MANUFACTURE.

United States Patent PROCESS FOR REDUCING DIETHYLENE GLYCOL FORMATION IN POLY(ETHYLENE TEREPH- THALATE) PREPOLYMER Kenneth T. Barkey, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y.

N0 Drawing. Original application Dec. 2, 1971, Ser. No. 204,365, now Patent No. 3,749,697. Divided and this application Dec. 7, 1972, Ser. No. 313,046

Int. Cl. Cg 17/013 U.S. Cl. 252-431 C 3 Claims ABSTRACT OF THE DISCLOSURE Formation of contaminant diethylene glycol by-product during manufacture of poly(ethylene terephthalate) prepolymer by ester exchange of dimethyl terephthalate and ethylene glycol in the presence of a catalyst mixture is minimized by including lithium acetate dihydrate as an essential ingredient in the catalyst mixture when the catalyst mixture consists of zinc acetate dihydrate or antimony trioxide, alone or together, as the catalyst or catalysts for said manufacture.

This is a division of application Ser. No. 204,365, filed Dec. 2, 1971, now Pat. No. 3,748,697..

FIELD OF THE INVENTION The preparation of polyesters such as poly(ethylene terephthalate) and poly(propyleneterephthalate) by ester exchange and condensation is well known. U.S. Pats. 2,465,319, 2,727,881 and 3,488,382 and many others describe such processes in great detail. A common feature of a large number of these processes is that they begin with a lower dialkyl ester of a bifunctional dicarboxylic acid which is condensed with a bifunctional glycol, the glycol usually being used as such although it can be der conditions facilitating removal of the lower alkanol such esters being equivalents of the glycol. These processes involve the initial preparation, in the presence of a catalyst, of a substantially monomeric protopolymer under conditions facilitating removal of the lower alkanol formed from the lower dialkyl ester by ester interchange with the glycol. This is sometimes referred to as the first stage or the ester exchange step of the polyester preparation and can be conducted under a variety of conditions using may different types of apparatus. For example, the lower dialkyl ester and the glycol in a mole ratio beginning at about 1 to 1.5 up to about 1 to 10 are placed in a reaction vessel equipped with a packed col umn with the vessel being heated at a temperature which permits the lower alkanol to pass through the column, with the glycol being retained by the column and returned to the reaction vessel by refluxing, whereby the lower alkanol is theoretically removed during this first stage of the polyester preparation which results in the formation of a monomeric protopolymer.

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The catalysts which may be used in the preparation of polyesters according to the above processes include organic and inorganic compounds of metals such as titanium, manganese, antimony, zinc, tin, lead, calcium, cobalt, lithium, combinations thereof, etc., many of which have heretofore been utilized by those skilled in the art. The prior art, for example, discloses a great number of such catalysts, some of which are described in patents such as US. 2,465,319, US. 2,720,502, US. 2,727,881, US. 3,488,382 and others. Specific catalysts heretofore known and which may be utilized include tetraisopropyl titanate, titanium dioxide, zinc acetate, zinc acetyl acetonate, lead oxide, calcium oxide, lithium ethoxide, antimony trioxide, manganese oxides and the like. Generally the acetates, chlorides, nitrates, sulfates, oxides and alkoxides of one or more of the metals Zinc, manganese, tin, lead, titanium, antimony, cobalt and lithium are preferred. For example, the catalyst system of zinc acetate and tetraisopropyl titanate is Well suited to attain the desired reaction activity. The catalyst (or catalyst mixture) is gen erally utilized in a concentration of from about 0.002 percent to about 0.2 percent by weight of the reactants being condensed. Higher or lower percentages can also be employed. Generally from about 0.001 percent to about 0.05 percent catalyst can be advantageously employed. Preferred ester exchange catalysts include, for example, zinc acetate, manganous acetate, cobaltous acetate, lithium acetate, and tetraisopropyl titanate. However, many others are known.

During the first stage manufacture of the above type of monomeric protopolymer polyester, specifically poly (ethylene terephthalate) prepolymer, by means of catalytic systems using the above described catalysts, a byproduct compound, diethylene glycol (DEG) is formed and reacts with the polyester to form a copolyester which is randomly distributed in the polyester, which itself is a homopolymer, from which copolyester it cannot readily be removed. The copolyester has a lower melting point than does the parent homopolymer polyester and melts over a broader temperature range than does the polyester homopolymer. Furthermore, the copolyester nucleates and crystallizes difierently than does the polyester homopolymer. As a result, during subsequent operations involved in the manufacture of a film product from the polyester product containing the copolyester as a contaminant by-product, many problems arise which have an adverse effect upon the quality and uniformity of the film product made therefrom and upon the processing operations necessarily used in making the film. For example, in the manufacture of the film product, the first stage polyester product is first melted and then cast to form a readily grindable solid product for use in further polymerization steps. Uniform nucleation and rapid crystal growth are essential for casting of such polyester prepolymer in an opaque readily disintegratable solid form. Presence of the copolyester contaminant in excessive and often varying amounts causes the cast polyester prepolymer to solidify in glassy, amorphous form which is difiicult to prebreak or crush, grind and fluidize for the subsequent further polymerization of the polyester prepolymer to a higher degree of polymerization in a sec ond polymerization stage in a known manner. The adverse effects of the copolyester contaminant are carried over into the melt from which the film product ultimately is made when the completely polymerized polyester is extruded. Adverse effects such as increased incidents of tearoffs and breaks are encountered during tentering and drafting operations during which the polyester film is biaxially oriented. Prior to the present invention, the only known way to minimize the latter forms of defects was to increase the inherent viscosity (I.V.) of the individual batches of polyester during the second stage of polymerization, thereby causing longer processing times and consequent loss of production capacity.

SUMMARY OF THE INVENTION The present invention comprises the use in the ester exchange step of polyester preparation of poly(ethylene terephthalate) from dimethyl terephthalate and ethylene glycol of an improved catalyst mixture wherein lithium in the form of a salt of an organic acid, e.g. lithium acetate dihydrate, is used in a minor bulfering amount in a two or three part catalyst system with catalytic amounts of zinc, antimony or zinc-antimony catalyst. Preefrably, the improved catalyst mixture consists essentially of lithiumzzinczantimony in the ratio by parts per million (p.p.m.) of dimethyl terephthalate (DMT) charged to the reactor of about 20:65 :300, preferably in the forms of lithium acetate dihydrate, zinc acetate dihydrate, and antimony trioxide, respectively. In the latter forms, the amounts of the ingredients used in the catalyst mixture, taken as percents of the weight of DMT charged, will be about 0.03% lithium acetate dihydrate, about 0.02% zinc acetate dihydrate and about 0.035% antimony trioxide, or about 0.12 part lithium acetate dihydrate, 0.08 part zinc acetate dihydrate, and about 0.14 part of antimony trioxide per 388 parts of dimethyl terephthalate charged to the reactor.

The polyester prepolymer formed as a result of the practice of the present invention is characterized in that the amount of diethylene glycol present in the first stage polyester product is advantageously lower than that present in corresponding prior art product, and the melting point of the first stage polyester prepolymer product is significantly higher than that of the corresponding prior art product, using in each prior case a catalyst mixture consisting of zinc and/or antimony compounds. The polyester prepolymer product is characterized in that it has advantageously improved casting, grinding and fluidization properties for the further manufacture of poly- (ethylene terephthalate) powder, which powder subsequently is melt extruded into film form, drafted and ten tered to form a biaxially-oriented poly(ethylene terephthalate) film support in a known way for use in manufacture of photographic film products.

The following examples illustrate the advantages of a preferred embodiment of the invention over the prior art practice.

TABLE II Process data and properties of poly (ethylene terephthalate) prepolymer Example 1 Example 2 Time to com lete ester exchange reaction to form prepo yrner, min 44 26 Inherent viscosity of prepolymer. 0. 44 0. 43 Carbpxyl radical content, meq./kg. 86 14 Melting point C 256 260-261 Diethylene glycol content, mole percent oi prepolymer 3. 5 1. 5

Thus, it is evident from comparison of the process data and properties of poly(ethylene terephthalate) prepolymers (Table II) made by the prior art method (Example 1) in the absence of lithium acetate dihydrate in the catalyst mixture and the method of the present invention (Example 2) wherein lithium acetate dihydrate is present in the catalyst mixture that the use of only a minor amount of lithium acetate dihydrate (Table I, Example 2) has reduced the reaction time for the esterexchange reaction by 18 minutes, from 44 minutes to 26 minutes. Also, while the inherent viscosity of the prepolymer polyester has not been significantly affected, i.e. 0.44 and 0.43, the carboxyl radical content, which under normal circumstances is indicative of presence of diethylene glycol, has been reduced greatly, from 36 to 14 mole equivalent (meq.) per kilogram (kg.) of prepolymer polyester. Further, the melting point of the poly(ethylene terephthalate) prepolymer (prepolymer polyester) has advantageously been raised from 256 C. to 260-261 C. by practice of the present invention. And, finally, the actual amount of diethylene glycol present in said prepolymer polyester has been reduced by two full percentage points, i.e. from 3.5 mole percent to 1.5 mole percent in the prepolymer polyester. In practice of the invention, it has been found that lowering of the mole percent of diethylene glycol present by one mole percent results in an increase in melting point of the polyester prepolymer by 2.15 C. In the present example, a reduction of 2 mole percent in diethylene glycol content of the polyester prepolymer has resulted in a 4 to 5 C. increase in melting point. The prepolymer polyester thus has physical property characteristics which one skilled in the art of poly-condensing the polyester prepolymer to the polyester and the manufacture of the so-obtained poly(ethylene terephthalate) into film form by known prior art methods will readily recognize as advantageous for the forming of biaxially-oriented film therefrom.

Procedures for carrying out the polycondensation process and procedures for forming biaxially-oriented film from poly(ethylene terephthalate) are described, for example, in US. Pats. 3,043,564 and 3,048,564, and such procedures may be used to make the polyester and to make biaxially-oriented film from the poly(ethylene terephthalate) of the present invention. When the poly(ethylene terephthalate) of the present invention is being tentered and drawn for the making into film, the adverse elfects discussed above, e.g. tearoifs and breaks, are found to be absent or considerably reduced in frequency.

While the amount of lithium used in practice of the invention has been shown in Example 2 to be 21 p.p.m. of lithium as Li (equivalent to 0.1176 g. of lithium acetate dihydrate) in a reaction mixture of two gram moles of dimethyl terephthalate with four gram moles of ethylene glycol, resulting in the formation of two gram moles of poly(ethylene terephthalate) prepolymer, the advantageous results of the invention also may be obtained by the use of from about 15 to about 60 p.p.m. of lithium as Li (equivalent to about 0.078 g. to about 0.340 g. of lithium acetate dihydrate) in a reaction mixture of two gram moles (388 g.) of dimethyl terephthalate with four gram moles (248 g.) of ethylene glycol. The amounts of antimony catalyst then used may range from about p.p.m. to about 500 p.p.m. (equivalent to from about 0.046 g. to about 0.230 g. of antimony trioxide) in the same reaction mixture. The amounts of zinc catalyst then used may range from about 32 p.p.m. to about p.p.m. (equivalent to from about 0.042 g. to about 0.168 g. of zinc acetate dihydrate) in the same reaction mixture.

Preefrably, the improved catalyst mixture of this invention will consist of 65 p.p.m. of zinc, 300 p.p.m. of antimony and from 21 to 35 p.p.m. of lithium in the form of the equivalent amounts of zinc acetate dihydrate, antimony trioxide and lithium acetate dihydrate, respectively, based on the weight of dimethyl terephthalate charged to the reactor.

EXAMPLE 3 The advantageous effect of the use of a minor amount of lithium in the zinc-antimony catalyst mixture has been found to be obtained also when recycled ethylene glycol from the poly(ethylene terephthalate) prepolymer recov-' ery system is used for manufacture of more of the polyester prepolymer with a new supply of dimethyl terephthalate.

Recycled ethylene glycol recovered from a previous poly(ethylene terephthalate) prepolymer reaction mass is used in the ester exchange reaction in a subsequent batch.

From the data, it is to be noted that inclusion of lithium in the catalyst mixture again caused lesser amounts of diethylene glycol to form (higher melting point) and lesser polyester acidity as evidenced by lower carboxyl content (15 meq/kg. of COOH versus 29 meq./kg. of COOH) Water (2-4 percent) is a major contaminant of such eth- 5 for the poly( y tefephthalate) p y ylene glycol. A high melting point for the polyester pre- EXAMPLE 5 polymer is indicative of low diethylene glycol content in the prepolymer. The melting point raising effect of the use The recovery of Waste polyfiethylene terephihalate) of lithium (as lithium acetate dihydrate) in the catalyst 10 from i manufaqunng ugeratlons ecqloglFal and system for the preparation of poly(ethylene terephthalate) economlc'fll necesslty' Practlce the mventlon Such prepolymer from recycled ethylene glycol along with new recovery Shown by the followmg data dimethylene terephthalate is shown in the following lab- Scrip poly(ethylene tereRhthalate) from fi t oratory batches of Table factunng operations was sub ected to glycolysls reactions 15 to obtain polyester of lower molecular weight for reproc- TABLE IV esslng to film grade poly(ethylene terephthalate). Various P.p.m. catalyst Prepolymer Melting catalyst systems were used to accelerate the breakdown 8%: of the polyester and to minimize diethylene glycol formation. The scrap poly(ethylene terephthalate) had the fol- 65 300 0 0'43 256 lowing characteristics before glycolysis: 3? 383 3 313 TABLE VI 2b 300 21 9 Inherent viscosity (I.V.) 0.65 ,ggggggg g: gg %f P Y Q d 35 8 Charged as lithium acetate dihydrate. 25 Meltmg p m C. 257 From the data it is seen that the use of Li (lithium ace- In ch f the following b tche A D, 300 of polytate dihydrate) gave poly(ethylene tercphth l te) p p y- (ethylene terephthalate) were reacted with 450 g. of ethmer melting 2 C. higher in each batch (1b and 2b) Where ylene glycol (1.5 Weight ratio of ethylene glycol-to scrap it was present as compared to those batches where lithium poly (ethylene terephthalate)) at the boiling point (198- Wa absent and and is indicative o the fact that 205 0.). Times necessary to give clear solutions varied 168$ diethylehe glycol Was formed even With Water Present depending upon the catalyst system and its concentration. in the glycol. After the scrap had been dissolved and reacted to low The advantageous (fleets 0f the Presence of lithium in molecular weight oligomers, excess ethylene glycol was t Catalyst mixture the content of contamdistilled oil and the melt was polymerized for two hours mant diethylcne glycol 1n h p p y er polyester have at 260 c. and at a pressure of less than 100 microns 0.1 also been found to be obtained when llthium is used in mm H Th resulting l h l terephthalate) combination with zinc as the only other catalytic matepolymer was cast and analyzed The resulting data are rial in the catalyst system or in combination with anti- Shown i T bl V11, mony as the only other catalytic material in the catalyst system. Examples 4 and 5 further illustrate such advan- 40 TABLE VII tageous results obtained by practice of the invention. In After two hrs. ot'melt the examples it is to be noted again that the obtainment mlymemamn of a higher melting point for the prepolymer polyester is Glycolysls, y, indicative of the lowering of the diethylene glycol content Batch Zn! LU mmutes COOH O in the pglygste g A 0 0 16 0.15 67 225 EXAMPLE 4 3:13:31: 3 53 32 31%? 3; it? Distilled monomer (250 g.) (i.e. poly(ethylene tereph- D 52 30 91 245 thalate) prepolymer) separated from poly(ethylene ter- 8 1:5233:gffig f sgg ggg gzi g ephthalate) prepolymer in a recovery still and containing 280 p.p.m. of zinc and 150 p.p.m. of antimony catalysts To eliminate the elfect of low molecular weight, as (based on weight of monomer charged) was reacted addievidenced by the inherent viscosity (I.V.) of the pretionally with new dimethyl terephthalate (388 g.) and polymer and its melting point, batches B and D were additional new ethylene glycol. In batch a, 300 p.p.m. polymerized further by solid phase buildup at 210 C. of Sb were added (as Sb O in batch b, 21 p.p.m. of 55 for seven hours. The results obtained are shown in Table Li (charged as lithium acetate dihydrate) and 300 p.p.m. VIII.

TABLE Vm 0 hour 1 hour 2 hours 5 hours 7 hours P.p.m. catalyst COOH, oooH, COOH, COOH, COOH, M.P., Zn Li 1 I.V. meq./kg. I.V. meqJkg. LV. meq./kg. LV. meq./kg. LV. meqJkg. 0.

Batch No B 162 0 0.18 137 0.26 77 0. 31 as 4s 0. 41 53 247 i D 102 52 0.15 91 0.21 64 0.27 54 0. 34 4e 0. as as 263 1 Charged as zinc acetate dihydrate. 2 Charged as lithium acetate dihydrate. of Sb (as Sb O were added. The results obtained are From the data in Table VIII, it is to be noted that shown in Table V. after polymerization for 7 hours in each batch, batch D, TABLE V which had 52 p.p.m. of lithium present along with 162 Rpm added Propmies of p.p.m. of zinc in the catalyst mixture, resulted in a recatalysts prepolymer cycled poly(ethylene terephthalate) product having COOH MP" COOH content of 38 meq./kg., compared to original L1 Sb I.V. (meq.lkg.) C. COOH content of 35 meq./kg. in the scrap polyester Batch number: and a melting point of 253 compared to the original a 0 300 0.42 29 256 of 257 C. The results from batch B show that in the b 21 300 15 258 absence of lithium, the zinc catalyst alone gave poly- Inherent visc s yester product which had 53 meq./kg. of COOH (com- 7 pared to 38 for D) and a melting point of only 247 (7.5 mole percent DEG) compared to 253 C. mole percent DEG). Therefore, it is evident that use of lithium acetate dihydrate in the catalyst mixture is advantageous for the scrap polyester recovery process as well as for the manufacture of fresh poly(ethylene terephthalate) prepolymer. The recycled prepolymer is in a form suitable for polycondensation to a poly(ethylene terephthalate) which can be made into a useful film product.

As used throughout this description, it is to be understood that the inherent viscosity (I.V.) of poly(ethylene terephthalate) prepolymer is in the range of about 0.40 to 0.44 and the inherent viscosity of polycondensed polyester, i.e. poly(ethylene terephthalate) is in the range of about 0.60 to 0.65.

Although the nature of the activity of the lithium acetate dihydrate in inhibiting the formation of diethylene glycol in accordance with the invention is not definitely known, it is believed that the acidity of the reaction mass is in some manner affected. However, when experirnents similar to those given in the examples were carried out using sodium acetate as a substitute for the lithium acetate in the catalyst mixture of the invention the results obtained were not as good and, in addition, the biaxially-oriented films made from the sodium acetate experiments were found to be hazy and not clear as were those obtained when lithium acetate dihydrate was used in the catalyst mixtures of the invention.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

I claim:

1. A catalyst mixture for manufacturing by an ester exchange process a polyester prepolymer of dimethyl terephthalate and ethylene glycol which process comprises heating in a reactor a liquid mixture of said ethylene glycol and said dimethyl terephthalate in the presence of an ester exchange catalyst at an elevated temperature for a period of time suflicient to form poly(ethylene terephthalate) prepolymer substantially free of diethylene glycol by-product, said catalyst mixture consisting essentially of a mixture of at least two ingredients selected from the following ingredients in the following amounts: (a) from about 15 to about parts per million of lithium, (b) from O to 500 parts per million of antimony, and (c) from 0 to about 130 parts per million of Zinc based on the weight of dimethyl terephthalate charged to the reactor, said zinc and lithium being present in said mixture in the forms of their acetate dihydrates and said antimony being present in said mixture as antimony trioxide.

2. The catalyst mixture according to claim 1 wherein said mixture consists essentially of from about 15 to about 60 p.p.m. of lithium and from about to 500 p.p.m. of antimony.

3. The catalyst mixture according to claim 1 wherein said mixture consists essentially of from about 15 to about 60 p.p.m. of lithium and from about 32 to about p.p.m. of zinc.

References Cited UNITED STATES PATENTS 3,068,204 12/1962 Perry et al. 252--43l 3,428,587 2/1969 Piirrna 26045.85 R 3,503,899 3/1970 Hergenrother 252-431C 3,651,017 3/1972 Tanabe et a1. 252430 DANIEL E. WYMAN, Primary Examiner A. P. DEMERS, Assistant Examiner US. Cl. X.R.

27 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,806 ,468 Dated June 25 197M Kenheth T. Barkey It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Inventofls) "der conditions facilitating Column 1, line53, delete and insert used in the removal of the lower alkanol" form of lower alkanoic acid ester thereof Signed and sealed this 1st day of October 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Commissioner of Patents Attesting Officer 

